Fuel cells are electrochemical devices designed to convert the high energy density of chemical bonds into electricity. The proton exchange membrane (PEM) fuel cell is the most prominent low-temperature fuel cell technology. These devices typically employ a polymer electrolyte membrane with pendant acidic groups as a method of transporting protons between the anode and cathode. However, the high proton concentration in PEM fuel cells creates a corrosive acidic environment in which only platinum group catalysts are stable, severely hindering commercial feasibility.
Anion exchange membrane (AEM) fuel cells, which transport hydroxide ions as opposed to protons, overcome this limitation by nature of an alkaline operating environment in which earth-abundant catalysts, such as nickel and manganese derivatives, exhibit suitable activity and stability. The AEM acts as a semipermeable separator, simultaneously transporting hydroxide anions, while preventing fuel crossover between the anode and the cathode. Slow hydroxide transport leads to significant ohmic losses, while poor fuel separation limits the cell's electrochemical potential. An ideal AEM would therefore be characterized by an efficient, percolating hydroxide transport network supported by a robust, dimensionally stable matrix.
While great strides have been made over the past decade, AEM fuel cells continue to perform unfavorably compared to PEM fuel cells. The lower performance can be partially attributed to the relatively low ionic conductivities of current AEMs, where the typical hydroxide conductivity of reported AEMs is often an order of magnitude lower than the proton conductivity of Nafion, the de facto standard commercial PEM. Thus, despite the cost advantage of AEM fuel cells, their lower performance has limited their commercial viability.
Given the high performance of PEMs, it is no surprise that their structure and chemistry have heavily influenced AEM design. As in PEMs, the general motif for synthesizing AEMs has been to attach pendant ionic salts along a robust hydrophobic polymer backbone. This approach typically manifests as aryl- or benzyl-substituted cations along an aromatic polymer chain. Whereas sulfonate is the pendant counter-anion of choice for PEMs, the pendant counter-cation in AEMs has been more varied with the aim of improving hydroxide conductivity and alkaline stability. Various approaches have involved membranes based on quaternary ammonium, imidazolium, guanidinium, phosphonium, and sulfonium cations, amongst others. Quaternary ammonium based on trimethylamine has been introduced on a variety of different polyaromatic backbones, including polysulfone, poly(phenylene oxide), and poly(etheretherketone). However, in these systems, the close proximity of the pendant cation to the rigid polymer backbone inhibits the formation of strongly segregated hydrophilic-hydrophobic domains. Consequently, these membranes are often characterized by poorly defined water-rich phases, leading to ion transport occurring in highly constricted and tortuous pathways.
The present invention addresses at least some of the current issues with exchange membranes and provides an anion transport membrane with a different morphology enabling efficient anion transport.